Terence H. O'Brien, Civil Defence, U.K. Official History of the Second World War, Civil Series, 1955. British Government Crown copyright has expired and it is now in the public domain. The 181 pages of relevance to modern civil defence only have been scanned in. The omitted material concerns trivia like WWII black-out curtains, which is no longer of interest to civil defence against enemy attacks and so has been excluded. Modern missile and bomber technology is not dependent for navigation upon lights shown in cities, therefore the extensive civil defence "black out" precautions are no longer of relevance (e.g. black curtains in windows to absorb light, slit covers on car headlights, and the use of air raid wardens to ensure that no lights are showing during air raids and to fine offenders). In addition, extensive material on relatively minor bombing air raids which is not of use for civil defence today has been excluded. Those who are interested in the history of trivial aspects can find the full book on the shelf at the U.K. National Archives in Kew, or in the British Library.
For declassified information on British civil defence research and war plans since 1945, please see https://archive.org/details/WarPlanUK
Please see also: The effects of the atomic bomb on Hiroshima, Japan (the secret U.S. Strategic Bombing Survey report 92, Pacific Theatre) located at: http://archive.org/details/TheEffectsOfTheAtomicBombOnHiroshima , http://archive.org/details/WarPlanUK , https://archive.org/details/civildefence and https://archive.org/details/Anderson_shelter